1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an electrophotographic image forming apparatus incorporating a thermal fixing device that fixes toner images onto recording media with a heated fixing member.
2. Discussion of the Background
In electrophotographic image forming apparatuses, such as printers, photocopiers, facsimiles, and multifunctional machines incorporating several of these functions, a fixing device is used to fix toner images in place on recording media such as sheets of paper. Typically, an electrophotographic fixing device includes a fixing member such as a belt or roller to receive recording media thereon, and a heater to heat the fixing member from within to fuse toner images onto the recording media, as well as a temperature controller to control operation of the heater by regulating power supplied thereto. In order to maintain a constant operational temperature in the fixing device, the temperature controller upon startup directs the heater to initially warm the fixing member up to a target temperature sufficient for fixing, and retain the heat in the fixing member until a recoding medium enters the fixing device.
Two important requirements of temperature control in such a thermal fixing device are the ability to rapidly raise the temperature of a fixing member to a desired target temperature, and the ability to prevent the temperature of the fixing member from overshooting the target temperature once that target temperature has been reached. The rapid heating requirement arises since an electrophotographic printer cannot operate unless the fixing device is sufficiently warm, in which taking much time to warm up the fixing member results in a longer period of time during which a user must wait for a print job to be executed. On the other hand, the overshoot prevention requirement should be met since overheating the fixing member leads to image defects due to fusing toner at excessively high temperatures, such as lack of gloss on printed images, or undesirable transfer of melted toner to recording media (often referred to as “hot offset”).
As can be readily appreciated, these requirements are mutually contradictory, however. That is, increasing power supply to the heater to accelerate the heating results in a greater amount of overshoot in the fixing temperature, and reducing power supply to the heater to prevent overshoot results in longer periods of time required to heat the fixing member to the target temperature.
To satisfy both of the above requirements, various methods have been developed to offer an efficient temperature controller for a fixing device, some of which employ on-off control and PID (control composed of proportional (P), integral (I), and derivative (D) actions), the two basic algorithms often used to control temperature in a thermal process.
Specifically, an ordinary on-off temperature controller works by turning on or off power supply to a heater depending on whether a process temperature is below or above a set-point temperature. When used in a fixing device, the on-off controller allows for an extremely short warm-up time, supplying the heater with full power as long as the fixing temperature remains below a desired operational temperature. However, such control fails to prevent an overshoot of the fixing temperature because the heater power turns off only after the fixing temperature exceeds the operational temperature.
By contrast, a PID controller controls a process temperature by adjusting power supply to a heater as a proportion of time during which the heater is active (referred to as “duty cycle”) according to a difference between the process temperature and a set-point temperature. When used in a fixing device, the PID controller maintains the heater power relatively high when the fixing temperature is farther below the set-point temperature, and decreases the heater power as the fixing temperature approaches the set-point temperature. Such control effectively reduces the amount of overshoot in the fixing temperature, but simultaneously results in an increased warm-up time compared to that required for warm-up with an on-off controller.
Hence, on-off control and PID control each has both advantages and drawbacks. A comparison between the two control techniques is shown in FIG. 1, which is a graph plotting a temperature T of a fixing member and a duty cycle D of a heater in a fixing device, both against time. The measurements of FIG. 1 are obtained with an on-off controller (“Ton-off” and “Don-off”) and a PID controller (“Tpid” and “Dpid”) controlling the heater to warm the fixing device to an operational set-point To.
As shown in FIG. 1, the operational temperature To is reached more rapidly with the on-off controller than with the PID controller, while the amount of overshoot is smaller with the PID controller than with the on-off controller.
Several conventional methods propose a temperature controller that can operate in either an on-off mode or a PID mode to combine the advantages of the two types of temperature control. Such a dual-mode temperature controller switches the control mode when a process temperature monitored by a sensor exceeds a switching threshold temperature.
For example, one conventional temperature control method for a fixing device controls operation of a heater using a combination of an on-off mode and an integral (I) control mode, which activates the heater continuously in the on-off mode as long as the monitored temperature remains below a switching threshold lower than an operational set-point, and enters the I-control mode to execute an integral control action when the process temperature exceeds the threshold temperature.
Other similar methods include a temperature control circuit that executes a PID control action when the process temperature exceeds the threshold temperature, as well as a temperature control method and apparatus that executes a proportional (P) control action when the switching threshold is exceeded.
Further, a sophisticated form of such dual-mode temperature control uses a combination of an on-off mode and a PID mode with multiple temperature thresholds. In addition to being capable of switching between the off mode and the PID mode at a switching threshold, this temperature controller can modify a tuning parameter of a PID algorithm when the process temperature exceeds each of the multiple temperature thresholds. Such a control method overcomes limitations of the preceding temperature controllers that only switch control mode at a single threshold temperature, and therefore can be insufficient where precision is needed to meet both rapid heating and overshoot reduction requirements in a thermal fixing device.
Owing to the combined advantages of on-off control and PID control, the dual-mode temperature controllers effectively provide both rapid heating and overshoot reduction where the fixing temperature continuously increases from a lower level (e.g., during initial warm-up). However, such a strategy does not work well in certain situations where the fixing temperature fluctuates toward a set-point rather than continuously increasing thereto. The following describes a detrimental situation for a conventional dual-mode temperature controller of a thermal fixing device.
FIG. 2 schematically illustrates a fixing device 120 used in a typical image forming apparatus.
As shown in FIG. 2, the fixing device 120 includes an endless fixing belt 124 running around a fixing roller 122 and a heat roller 123, with a pressure roller 121 pressed against the fixing belt 124 to form a fixing nip therebetween. The fixing device 120 also includes first and second heaters 130 and 131 inside the heat roller 123 and the pressure roller 131, respectively, as well as a temperature sensor 125 monitoring a temperature of the fixing belt 124 adjacent to the heat roller 123.
During operation, the heaters 130 and 131 heat the fixing belt 124 according to a belt temperature T sensed by the temperature sensor 125 so as to maintain the temperature T at desired levels. When the image forming apparatus receives a print request, the fixing belt 124 rotates in sync with the pressure roller 121 to pass a recording sheet through the fixing nip so as to apply heat and pressure to the incoming recording sheet.
FIG. 3 provides a graph showing the belt temperature T monitored by the temperature sensor 125 in the fixing device 120 plotted against time in seconds (s), together with the operating status of the fixing belt 124 since startup of the image forming apparatus.
As shown in FIG. 3, during an initial warm-up phase Pw, the fixing belt 124 rotates with the pressure roller 121 while heating up to a standby temperature Ts sufficient for fixing with the heaters 130 and 131 activated. When no print request is received upon completion of the warm-up phase Pw, the fixing device 120 enters a standby phase Ps in which the fixing belt 124 and the roller 121 stop rotation while the heaters 130 and 131 remain active to maintain the belt temperature T at the constant level Ts, holding it ready for rapid recovery.
When receiving a print request during the standby phase Ps, the fixing device enters a recovery phase Pr in which the fixing belt 124 and the roller 121 resume rotation so that the heaters 130 and 131 uniformly heat the entire length of the rotating fixing belt 124 to an operational temperature To sufficient for fixing, which is in this case slightly lower than the standby temperature Ts. When the operational temperature To is reached, the fixing device 120 enters a fixing phase Pf to fuse a toner image onto an incoming recording sheet. After fixing, the fixing device 120 again enters the standby phase Ps by stopping rotation of the fixing belt 124 and the roller 121.
FIG. 4 illustrates in detail the belt temperature T monitored from the standby phase Ps to the fixing phase Pf.
As shown in FIG. 4, the belt temperature T sharply declines from the standby temperature Ts upon switching from the standby phase Ps to the recovery phase Pr, and thereafter fluctuates between higher and lower levels while gradually approaching the set-point temperature To. Such fluctuation of the monitored temperature T arises from uneven distribution of heat over the length of the fixing belt 124. That is, the fixing belt 124 during standby has relatively hot portions retained in contact with the rollers 123 and 121 and receiving heat from the heaters 130 and 131 therethrough, and relatively cold portions not in direct contact with the heaters 130 and 131. When the unevenly heated belt 124 rotates after standby, the temperature sensor 125 senses temperatures of the (relatively) hot and cold portions alternately so that its output fluctuates between higher and lower levels during recovery. Specifically, the belt temperature T fluctuates below the operational set-point To with a certain difference between the highest and lowest levels (e.g., on the order of approximately 20 degrees), where the standby set-point Ts is set at a temperature equal to or slightly (e.g., on the order of approximately 10 degrees) lower or higher than the operational set-point To.
As mentioned, the conventional dual-mode temperature controller switches the control mode when the monitored fixing temperature reaches a threshold temperature. Such a switching threshold is set at an appropriate level depending on properties of the fixing device, such as the thermal capacities of fixing members, and the dead time required until the fixing temperature starts to rise upon activation of the heater, which typically falls within a range approximately 20 to 50 degrees lower than a desired operational temperature.
With further reference to FIG. 4, consider a case where the switching threshold is set at a temperature Tx between the highest and lowest levels of the belt temperature T during recovery. Naturally, the fluctuating temperature T reaches the switching threshold Tx more than once, and the temperature controller switches the control mode frequently whenever the threshold temperature Tx is reached. The result is the recovery phase Pr is longer than required, reducing the efficacy of the dual-mode temperature controller in rapidly heating the fixing member.
Hence, what is required is a temperature controller for a fixing device which provides both rapid heating and reliable overshoot prevention even when a monitored fixing temperature fluctuates during recovery from standby. Having such a stable temperature controller is advantageous particularly with modern fixing devices that employ thin-walled fixing rollers or fixing belts with low thermal capacities for reducing warm-up time and energy consumption, which are ready to warm up and to cool down, and therefore are susceptible to temperature variations.